In the field of radiation therapy, e.g. for the treatment of tumors, it is understood that hadron radiation therapy (notably with protons or ions, such as carbon-ion therapy) offers significant advantages over X-ray or gamma ray therapies. In general hadron radiation therapy is an upcoming cancer treatment in which a hadron beam is used for irradiation. The number of hadron radiation therapy centers, mainly equipped with a proton radiation therapy installation, is growing rapidly and world wide over 50000 patients have been treated up to now.
The dominant processes by which protons and other hadrons deposit energy in tissue are atomic ionization and excitation. Most of the kinetic energy is deposited in the Bragg Peak (BP) at the very end of the track. Hadron radiation therapy has the potential to realize an attractive dose conformation, thus sparing the healthy tissue surrounding the tumor. This allows for either dose escalation for hypoxic tumors or for fewer side effects for nearby organs at risk. These are major advantages in the treatment of tumors located in critical parts of the body, such as the brain, e.g. near the eye.
Many recently commissioned hadron radiation therapy systems use the spot scanning technique wherein magnetic fields are used to steer the radiation beam in the desired direction. A pencil beam is stepped or scanned many times over the tumor or other defined treatment field of the target, with the energy and intensity being varied so that the dose in each microvolume of the tumor can be optimized. The beam intensity is continuously controlled. Not the entire tumor is irradiated at the same time but the irradiation is done spot after spot and slice after slice.
It is known to carefully plan a radiation session by making CT images of the patient, making calculations (often based on earlier phantom testing), etc. However, the actual position of the Bragg Peak within the actual patient depends among others on the characteristics of intermediate tissues, that may differ with the patient and with the irradiation position. Also anatomical differences may occur in the time span between planning and actual performance of the therapeutic radiation session, for example local changes of the patient anatomy, tissue composition, etc. It is therefore common understanding that deviations are present between the treatment plan and the actually applied radiation therapy. In particular with regard to the penetration depth of the beam into a human brain the actual position of the Bragg Peak is observed to deviate significant, e.g. between 10 and 15 millimeters, from the planned position. Critical tissues located near the tumor to be treated could receive an overdose, or the tumor may receive an underdose as a result.
It is therefore of great importance to verify the Bragg Peak position, preferably even during the hadron radiation therapy session, to be sure that the dose is delivered as planned.
Positron Emission Tomography (PET) is currently the only method effectively used in this field for dose verification. Isotopes that decay by positron emission are formed by nuclear reactions in the proton track and can be used for PET imaging, allowing to check the administered dose profile. However, the half-life of the suited isotopes is of the order the duration of the fraction or longer. Dose profile monitoring with PET within the time duration of a spot-step, commonly less than 0.1 s, therefore seems unlikely.
Research is currently also conducted into the use of prompt gamma rays for hadron beam penetration verification, monitoring, and possibly also for real-time control of the beam during irradiation of the target. Nuclear fragmentation reactions occur along the track in the target resulting in the emission of large numbers of neutrons and prompt gamma rays. These gamma rays arise from the statistical decay of nuclei exited at energies below the nuclear binding energy (8 MeV). Prompt gamma rays are a likely candidate for dose monitoring because their number is much larger than the number of PET isotopes. The absence of washout effects as seen with PET and the close correlation between the beam penetration depth range and the prompt gamma ray production position are important additional advantages. The correlation mentioned is the result of the fact that nuclear reactions occur up to the last few millimeters of the track where the hadrons energy falls below the Coulomb barrier threshold.
In WO2010/000857 several approaches of the use of prompt gamma rays are discussed, mainly aimed to work towards a real-time measurement of the spatial dose distribution in the target.
In one embodiment an Anger gamma camera is proposed wherein a collimator is placed directly over a flat panel detection crystal and a PMT array at the rear of the detection crystal. The collimator consists of a thick sheet of gamma ray blocking material, e.g. lead, with a multitude of adjacent holes through it. The camera is installed to have its optical axis at right angles to the beam axis. The PMT may be arranged in a two-dimensional array and it is envisaged that with the use of two such camera's a three dimensional image of the distribution of prompt gamma rays can be obtained.
In another embodiment WO2010/00857 proposes a pinhole gamma camera, wherein a single pinhole collimator is arranged at a distance in front of a single scintillation element type detector. The pinhole is configured and arranged so as to provide a field of view of the camera that encompasses the entire track of the proton beam within the target, so that there is no need to move the camera to observe the prompt gamma distribution along the track. Depending on whether a linear or two-dimensional array of PMTs is used, a one dimensional or two dimensional image of the prompt gamma distribution is obtained.
The present inventors consider the Anger camera and single pinhole gamma camera as proposed in WO2010/00857 unsuited as the collimator attenuates most of the incident gamma photons and thus greatly limits the sensitivity of the camera. The potential for real-time measurement of proton beam penetration depth is therefore severely restricted.
In a recent proposal it has been suggested to use in a hadron radiation installation a gamma camera with a slit collimator having an elongated slit aperture that is arranged substantially at right angles to the beam axis and with a 2D-detector. This detector has a single scintillation element embodied as a flat panel detection crystal and a 2D-array of photodetectors at the rear side of the crystal. The slit is configured as a knife-edge type slit with a fixed width. The collimator and slit thereof are arranged such with respect to the target that the camera has a field of view that encompasses the entire track of the beam within the target, so that there is no need to move the camera to observe the prompt gamma distribution along the track. As the slit aperture is greater in cross-section than the single pinhole and also provides less attenuation than the collimator in the Anger camera, the above-mentioned problems are potentially solvable.